Efficient UV-emitting phosphors based on cerium-activated calcium pyrophosphate and lamps containing the same

ABSTRACT

The present invention is directed to phosphors exhibiting efficient, narrow band UV emission, with peak emission at 330 nm (from excitation at 254). These phosphors are cerium-activated calcium pyrophosphate (beta-phase) of nominal formulation 
     
         Ca(.sub.2-w-x-y) (NSF).sub.w Ce.sub.x Na.sub.y P.sub.2 O.sub.7 
    
     wherein: 
     NSF is the non-stoicheometric factor; 
     0≦w≦0.1; 
     0.05≦x≦0.20; and 
     0.05≦y≦0.20.

BACKGROUND OF THE INVENTION

The present invention is directed to luminescent materials, to methodsof making such materials, and to methods of using such materials. Moreparticularly, the present invention is directed to efficient narrow bandultra-violet (UV) emitting phosphors, especially useful for erythemalapplications and as "black-lights".

Ultra-violet emitting phosphors are known in the art. See for example,U.S. Pat. Nos. 4,189,661 to Haugsjaa et al., 4,153,572 and 4,088,922 toWolfe, 4,070,583 to Rabatin, 4,224,553 to Hellwig, and GB Pat. No.1,565,811.

One such UV emitting phosphor is the GTE Sylvania Type 2011, which hasthe chemical formula BaSi₂ O₅ :Pb, and is available from the Chemicaland Metallurgical Division, GTE Precision Materials Group, Towanda, PA18848.

The present invention represents an alternative to the above-describedphosphors having advantages over previous UV-emitting phosphorsincluding; (1) a fluorescence in the UV spectral region at 330 nm, whoseintensity under 254 nm, excitation exceeds that of the Ba silicate: Pbphosphor by as much as 18%. (2) a more favorable absorption than theBa-silicate: Pb phosphor which permits it to also convert the 297 nm and313 nm UV lines emitted by the Hg plasma into 330 nm emission. (3) Anintense UV emission peaking at the wavelength location which shouldpermit efficient sensitization of visible and near impared emissionsfrom trivalent rare-earth cons, such as Tb³⁺, Dy³⁺, Tm³⁺ and Eu³⁺, andalso for sensitization of the technologically important emission fromMn²⁺.

SUMMARY OF THE INVENTION

The present invention is directed to phosphors exhibiting efficient,narrow band UV emission, with peak emission at 330 nm (from excitationat 254). These phosphors are cerium-activated calcium pyrophosphate(beta-phase) of nominal formula

    Ca.sub.(2-w-x-y) (NSF).sub.w Ce.sub.x Na.sub.y P.sub.2 O.sub.7

wherein:

NSF is the non-stoichiometric factor;

0≦w≦0.1;

0.05≦x≦0.20; and

0.05≦y≦0.20.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a corrected emission spectrum for a typical cerium-activatedcalcium pyrophosphate phosphor, prepared in accordance with the methodof the present invention (see Example 1) compared with the correspondingemission spectrum of a commercially available long wave-lengthUV-emitting phosphor, Sylvania Type 2011;

FIG. 2 is the excitation spectra of the phosphor of example 1 and theType 2011 phosphor, measured for emission at the peak of the respectivebands, showing the different spectral regions of emission-generatingabsorption for the two materials; and

FIG. 3 is a lateral view, partially cut-away, of a UV lamp utilizing aphosphor of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The emission spectrum for 254 nm excitation is plotted in FIG. 1, and iscontrasted with the emission spectrum of a commercial UV-emittingphosphor, GTE Sylvania, Type 2011. It is evident from the emissionspectra that on excitation at 254 nm (the wavelength of importance inlow-pressure fluorescent lamps) the emission intensity of thecerium-activated pyrophosphate phosphors of the present inventionapproach that of the commercial material.

The excitation spectra of FIG. 2, measured for emission at the peak ofthe respective bands, detail the different spectral regions ofemission-generating absorption for the two materials. Although theexcitation spectra cross around 254 nm, in accordance with thecomparable peak height of the emission bands for the two phosphors (seeFIG. 1), the excitation region for the cerium-activated material coversa wider portion of the UV spectrum, to the long-wavelength side of 254nm. As a result, the cerium-activated calcium pyrophosphate material ofthe present invention is capable of absorbing, and converting into 330nm emission, both the 297 nm and 313 nm UV lines emitted by a Hg plasma,which are not appreciably absorbed by the commercial phosphor.

The efficient conversion by the phosphors of the present invention of254 nm radiation into UV-emission by cerium, has important implicationsfor the use of such an intense Ce emission for the purpose ofsensitization of visible and near infrared emission from trivalentrare-earth ions, such as Tb³⁺, Dy³⁺, Tm³⁺, and Eu³⁺, and also for thesensitization of the technologically important emission from Mn²⁺.

Two types of firing atmosphere were tested in the synthesis of thepresent phosphor materials; a `CO atmosphere` (described infra) inExample 1 and a mixture of nitrogen and hydrogen `forming gas`. Althoughboth methods yield efficient UV emitting materials, those resulting fromthe `forming gas` firing were judged preferable on the basis of having amore intense 330 nm emission under similar (254 nm) excitationconditions.

All of the chemical raw materials employed in phosphor preparation wereof Luminescent or Reagent grade purity. A powder blend was created bymilling together the desired quantities of these raw materials. Thecomposition of the various powder blends, together with the processingsteps required for phosphor preparation are given in the example.

EXAMPLE 1

    ______________________________________                                        Reagent       Mole Ratio                                                                              Weight (grams)                                        ______________________________________                                        CaHPO.sub.4   1.74      37.88                                                 CeO.sub.2     0.10      2.75                                                  Na.sub.2 CO.sub.3                                                                           0.05      0.85                                                  (NH.sub.4).sub.2 HPO.sub.4                                                                  0.26      5.49                                                  ______________________________________                                    

After a thoroughly mixed power blend had been achieved it was placed inan alumina crucible. The crucible containing the powder blend was theninserted into a larger crucible and the intervening space between thetwo crucibles was filled with charcoal granules. Covers were placed onboth crucibles and they were placed in a furnace and heated to 1000° C.and held at temperature for a period of 1.0 hour. At the end of thefiring period the double crucibles were removed from the furnace andwere allowed to cool to room temperature before their covers wereremoved.

The resulting white bodied phosphor of nominal formula

    Ca.sub.1.74 Ce.sub.0.10 Na.sub.0.10 P.sub.2 O.sub.7

is isostructural with the low temperature (beta) polymorph of Ca₂ P₂ O₇.It has a violet emission when excited by 254 nm radiation, with a peakintensity at 330 nm of 110% relative to that of the BaSi₂ O₅ :Pbreference material.

EXAMPLE 2

    ______________________________________                                        Ingredient     Mole Ratio                                                                              Weight (gms)                                         ______________________________________                                        CaHPO.sub.4    1.78      19.37                                                Na.sub.2 CO.sub.3                                                                            0.04      0.34                                                 CeO.sub.2      0.08      1.10                                                 (NH.sub.4)HPO.sub.4                                                                          0.22      2.32                                                 ______________________________________                                    

The powder blend was placed in an alumina tray and fired at 1000° C. fora period of 1.0 hour in an atmosphere consisting of 95% (vol) Nitrogenand 5% (vol) Hydrogen (forming gas). After the prescribed firing periodhad elapsed, the powder was permitted to furnace cool to ≃400° C. in theforming gas atmosphere.

The resulting white bodied phosphor of nominal formula

    Ca.sub.1.78 Ce.sub.0.08 Na.sub.0.08 P.sub.2 O.sub.7

is isostructural with the low temperature (beta) polymorph of Ca₂ P₂ O₇.It has a violet emission when excited by 254 nm radiation, with a peakintensity at 330 nm of 110% relative to that of the BaSi₂ O₅ :Pbreference material.

EXAMPLE 3

    ______________________________________                                        Ingredient     Mole Ratio                                                                              Weight (gms)                                         ______________________________________                                        CaHPO.sub.4    1.74      18.94                                                Na.sub.2 CO.sub.3                                                                            0.05      0.42                                                 CeO.sub.2      0.10      1.38                                                 (NH.sub.4).sub.2 HPO.sub.4                                                                   0.26      2.75                                                 ______________________________________                                    

The powder blend was placed in an alumina tray and fired under identicalconditions to those described in Example 2 above. The resulting whitebodies phosphor of nominal formula

    Ca.sub.1.74 Ce.sub.0.10 Na.sub.0.10 P.sub.2 O.sub.7

is isostructural with the low temperature (beta) polymorph of Ca₂ P₂ O₇.It has a violet emission when excited by 254 nm radiation, with a peakintensity at 330 nm of 115% relative to that of the BaSi₂ O₅ :Pbreference material.

EXAMPLE 4

    ______________________________________                                        Ingredient     Mole Ratio                                                                              Weight (gms)                                         ______________________________________                                        CaHPO.sub.4    1.70      18.50                                                Na.sub.2 CO.sub.3                                                                            0.06      0.51                                                 CeO.sub.2      0.12      1.65                                                 (NH.sub.4).sub.2 HPO.sub.4                                                                   0.30      3.17                                                 ______________________________________                                    

The powder blend was placed in an alumina tray and fired under identicalconditions to those described in Example 2 above. The resulting whitebodied phosphor of nominal formula

    Ca.sub.1.70 Ce.sub.0.12 Na.sub.0.12 P.sub.2 O.sub.7

is isostructural with the low temperature (beta) polymorph of Ca₂ P₂ O₇.It has a violet emission when excited by 254 nm radiation, with a peakintensity at 330 nm of 118% relative to that of the BaSi₂ O₅ :Pbreference material.

EXAMPLE 5

    ______________________________________                                        Ingredient     Mole Ratio                                                                              Weight (gms)                                         ______________________________________                                        CaHPO          1.66      18.07                                                Na.sub.2 CO.sub.3                                                                            0.07      0.59                                                 CeO.sub.2      0.14      1.93                                                 (NH.sub.4).sub.2 HPO.sub.4                                                                   0.34      3.59                                                 ______________________________________                                    

The powder blend was placed in an alumina tray and fired under identicalconditions to those described in Example 2 above. The resulting whitebodies phosphor of nominal formula

    Ca.sub.1.66 Ce.sub.0.14 Na.sub.0.14 P.sub.2 O.sub.7

is isostructural with the low temperature (beta) polymorph of Ca₂ P₂ O₇.It has a violet emission when excited by 254 nm radiation, with a peakintensity of 330 nm of 112% relative to that of the BaSi₂ O₅ :Pbreference material.

EXAMPLE 6

    ______________________________________                                        Ingredient     Mole Ratio                                                                              Weight (gms)                                         ______________________________________                                        CaHPO.sub.4    1.62      17.63                                                Na.sub.2 CO.sub.3                                                                            0.08      0.68                                                 CeO.sub.2      0.16      2.20                                                 (NH.sub.4).sub.2 HPO.sub.4                                                                   0.38      4.01                                                 ______________________________________                                    

The powder blend was placed in an alumina tray and fired under identicalconditions to those described in Example 2 above. The resulting whitebodied phosphor of nominal formula

    Ca.sub.1.62 Ce.sub.0.16 Na.sub.0.16 P.sub.2 O.sub.7

is isostructural with the low temperature (beta) polymorph of Ca₂ P₂ O₇.It has a violet emission when excited by 254 nm radiation, with a peakintensity at 330 nm of 116% relative to that of the BaSi₂ O₅ :Pbreference material.

A second aspect of the present invention is illustrated in FIG. 3. Anultra-violet lamp 30 comprises a transparent, light-transmitting sealedenvelope 42, prepared from quartz, or preferably, glass. The envelope 42is fitted at each end with mounts comprising electrodes 40a and 40b,re-entrant stem presses 36a and 36b, and lead-in conductors 38a and 38b.Base caps 34a and 34b and pins 32a and 32b are provided at each end ofthe envelope, together with a small charge of mercury 44 within theenvelope 42. The inner surface of envelope 42 is coated with a phosphorlayer of the present invention 46.

The present invention has been described in detail, including thepreferred embodiments thereof. However, it will be appreciated thatthose skilled in the art, upon consideration of the present disclosure,may make modifications and improvements on the invention and still bewithin the scope and spirit of this invention as set forth in thefollowing claims.

What is claimed is:
 1. Efficient narrow band ultra-violet emittingphosphors consisting essentially of cerium activated calciumpyrophosphate beta-phase matrices having the general formula:

    Ca.sub.(2-w-x-y) (NSF).sub.w Ce.sub.x Na.sub.y P.sub.2 O.sub.7

wherein: NSF is the non-stoicheometric factor; 0≦w≦0.1; 0.05≦x≦0.20; and0.05≦y≦0.20;said phosphors being characterized by a peak emission atabout 330 nm when excited by short wave ultra violet light of 254 nm. 2.The phosphors of claim 1, wherein w is from 0 to 0.06.
 3. The phosphorsof claim 1, wherein x is from 0.08 to 0.16.
 4. The phosphors of claim 1,wherein y is from 0.08 to 0.16.
 5. The phosphor of claim 1, having theformula:

    Ca.sub.1.74 Ce.sub.0.10 Na.sub.0.10 P.sub.2 O.sub.7.


6. The phosphor of claim 1, having the formula:

    Ca.sub.1.78 Ce.sub.0.08 Na.sub.0.08 P.sub.2 O.sub.7.


7. The phosphor of claim 1, having the formula:

    Ca.sub.1.70 Ce.sub.0.12 Na.sub.0.12 P.sub.2 O.sub.7.


8. The phosphor of claim 1, having the formula:

    Ca.sub.1.66 Ce.sub.0.14 Na.sub.0.14 P.sub.2 O.sub.7.


9. The phosphor of claim 1, having the formula:

    Ca.sub.1.62 Ce.sub.0.16 Na.sub.0.16 P.sub.2 O.sub.7.


10. An ultra-violet lamp comprising:(a) a sealed tubular envelope; (b) adischarge-sustaining filling including mercury contained within saidenvelope; (c) electrodes disposed within said envelope and operable whenenergized to sustain a discharge therebetween to generate ultra-violetradiation of about 330 nm; and (d) a phosphor coating disposed on theinterior surface of said envelope, said phosphor coating having thenominal formula

    Ca.sub.(2-w-x-y) (NSF).sub.w Ce.sub.x Na.sub.y P.sub.2 O.sub.7

wherein: NSF is the non-stoicheometric factor; 0≦w≦0.1; 0.05≦x≦0.20; and0.05≦y≦0.20.
 11. The lamp of claim 10, wherein the phosphor has theformula:

    Ca.sub.1.74 Ce.sub.0.10 Na.sub.0.10 P.sub.2 O.sub.7.


12. The lamp of claim 10, wherein the phosphor has the formula:

    Ca.sub.1.78 Ce.sub.0.08 Na.sub.0.08 P.sub.2 O.sub.7.


13. The lamp of claim 10, wherein the phosphor has the formula:

    Ca.sub.1.70 Ce.sub.0.12 Na.sub.0.12 P.sub.2 O.sub.7.


14. The lamp of claim 10, wherein the phosphor has the formula:

    Ca.sub.1.66 Ce.sub.0.14 Na.sub.0.14 P.sub.2 O.sub.7.


15. The lamp of claim 10, wherein the phosphor has the formula:

    Ca.sub.1.62 Ce.sub.0.16 Na.sub.0.16 P.sub.2 O.sub.7.